Improvements to Heading component sizing about brand HOT 3 CLOSED

@rezrah I think the size prop feels weird that the values are separate from the values on the scale. If we use scale values could we use 400, 500, 800 etc.? Or even, use the pixel size values that we're going to be applying in Primer Primitives?

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@ashygee yes totally. I recall a conversation about the hundred naming convention being confusing, which is why I did it this way? Effectively the only options are 1-6.. which correspond to the default sizing or h1-h6. I can definitely move it over to that scale, but it does feel more abstract to me and requires assumed knowledge of that scale.

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@rezrah and I discussed this and I completely understand the single digits now. the number refers to the heading level size to be rendered. Therefore size="3" = the size used by default in an H3 (previously rendered in CSS with .h3-mktg)

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